1. Field of the Invention
The invention relates to a semiconductor device provided with a diode and an Insulated Gate Bipolar Transistor (IGBT).
2. Description of Related Art
A semiconductor device provided with a diode and an IGBT is known. Such a semiconductor element is typically referred to as a Reverse Conducting IGBT (RC-IGBT). FIG. 14 is a sectional view of a typical RC-IGBT. When the IGBT of the RC-IGBT is turned on, a snapback phenomenon occurs. FIG. 15 is a view showing a change in a collector current Ic when a voltage Vce between a collector and an emitter (hereinafter also referred to as simply “voltage Vce” or “collector—emitter voltage Vce”) is gradually increased, while a voltage of equal to or greater than a threshold value is being applied to a gate electrode 400 of an RC-IGBT in FIG. 14 (i.e., while a gate is on). When the voltage Vce is gradually increased, only a small amount of the current Ic flows, as indicated by the arrow 500 in FIG. 15. This is because electrons supplied from an emitter region 410 of the IGBT to a drift region 430 through a body region 420 (i.e., a channel) flow toward a cathode 450 of a diode, as indicated by the arrow 550 in FIG. 14. Therefore, the voltage applied to a pn junction 442 between the drift region 430 and the collector region 440 is lower than the collector—emitter voltage Vce. Therefore, as shown in FIG. 15, the pn junction will not turn on even when the voltage Vce reaches an on-voltage Vth of the pn junction. The voltage Vce increases to a higher voltage than the voltage Vth, as indicated by the arrow 500 in FIG. 15. When the voltage Vce increases to a predetermined voltage Vp, the voltage applied to the pn junction 442 at this point reaches the on-voltage Vth and the pn junction 442 turns on. That is, holes flow from the collector region 440 into the drift region 430, and electrodes flow via the collector region 440 as indicated by the arrow 560 in FIG. 14. When the pn junction 442 turns on, the voltage Vce suddenly decreases and the current Ic suddenly increases, as indicated by arrow 510 in FIG. 15. This kind of snapback leads to an increase in loss.
Japanese Patent Application Publication No. 2007-288158 (JP 2007-288158 A) describes technology that prevents this snapback phenomenon by adjusting parameters such as the thickness, width, and resistivity and the like of each semiconductor layer in an RC-IGBT. However, these parameters greatly affect other characteristics and manufacturing conditions and the like of the RC-IGBT. Therefore, these parameters cannot be set only to prevent the snapback phenomenon. The invention thus provides a semiconductor device that inhibits the snapback phenomenon.